This invention relates to high power RF amplifier systems, such as those employed in semiconductor plasma processing applications.
Plasma processing installations, which includes plasma reactors, are in widespread use in semiconductor fabrication. Plasma reactors are used to change the electrical characteristics of raw materials (e.g. silicon) during the manufacture of semiconductor-based electrical components like discrete transistors, medium and large scale integrated circuits, microprocessors and random-access memories. Typical operations performed using plasma reactors include sputtering, plasma etching, plasma deposition, and reactive ion etching.
In operation, a semiconductor work-piece is placed in the reactor. Next, gas is introduced into the plasma reactor at low pressure. Then, radio frequency (RF) power is applied to the gas to convert the gas into a plasma. The plasma is comprised of electrically-charged ions that react with exposed regions of the semiconductor work-piece. As a result of many such operations, electrical circuits are created in the semiconductor work-piece.
Plasma processing installations used in semiconductor fabrication generally comprise an RF generator, an RF power cable coupled at one end to the RF generator, an impedance matching network, and a RF power cable or a pair of copper straps which connect to the electrodes of the plasma reactor. During operation, the impedance of the plasma reactor is subject to substantial variation. Prior to ignition, the gas in the reactor is not ionized and therefore not conductive. Upon application of RF power, the gas begins to ionize and the load impedance drops as charge carriers are created in the reactor. After the start-up period a steady-state operating condition is eventually achieved.
Variations in the plasma flux (the product of the plasma density and the plasma charge velocity) can also cause significant transients in the load impedance, even during steady-state operations. In addition, during ignition and prior to achievement of steady-state, substantial impedance variations encountered may lead to significant power being reflected back to the RF generator causing it to become unstable and possibly destabilize the plasma process or damage the RF generator. This is especially the case for high xe2x80x9cQxe2x80x9d plasma processes. As will be further illustrated, standard non-dissipative filter configurations are not sufficient for the stable operation of the entire range of processes encountered.
In the art of plasma deposition or sputtering, for example, the process is driven by radio frequency energy typically provided at a relatively constant frequency, or band of frequencies such as for instance 13.56 MHz, xc2x15%, at levels up to several kilowatts. Typically, there is an RF generator coupled to a plasma chamber with a matching network interposed between them to match the impedance of the plasma chamber to the RF generator output source impedance, which is typically 50 ohms.
An RF energy delivery system may comprise an RF generator, a matching network, and a load. Frequency agile plasma systems, which operate over a pre-determined frequency bandwidth instead of a constant frequency, for instance a bandwidth representing a fluctuation of between about xc2x15 to xc2x110%, are becoming generally more desirable as they allow greater freedom to accomplish optimal plasma impedance match and thus allow a fixed or variable matching network.
Because the plasma does not behave like a linear ohmic resistance, the application of RF energy by the RF generator to the plasma chamber produces out of band energy which can be at multiples of the source frequency (harmonics) or at fractions thereof (sub-harmonics).
Dissipative filters have often been employed in communications work, for example, as a narrow bandpass I.F. filter after the first down converter of a receiver front end. Dissipative filters are employed to improve performance, where it is needed to provide proper controlled termination to the out of band signals. However, dissipative filters have not been favored because they do not offer the sharp attenuation slope of an equivalent lossless filter. Consequently, because of the shallow attenuation slope and energy dissipation problems, circuit designers have been reluctant to interpose filters of this type in an RF delivery system where the RF power can be several kilowatts.
A dissipative harmonic filter can be interposed in between the generator and the matching network to deal with the problem of out-of-band signals generated by the non-linearity of the plasma load as described in U.S. Pat. No. 5,187,457. Previous attempts to do this have generally involved reflective type lossless filters, which reflect rather than absorb harmonics. This, however, did not solve plasma chamber stability problems associated with specific process conditions, specifically high xe2x80x9cQxe2x80x9d recipes, because standard reflective filters with both Chebyshev or elliptic designs do not provide dissipative termination of harmonics. While these designs keep harmonics from the load, they ground harmonics via alternative paths. The harmonics in the ground paths create harmonic ground currents, known as xe2x80x9chot groundsxe2x80x9d which create additional gate-source voltage differential, potentially harmful to MOSFET dies. The presence of harmonic ground currents also create harmonically super-imposed fundamental frequency waveform at the MOSFET gates thereby affecting their switching characteristics. This results in inconsistent drive level requirements for the same output power at a given supply voltage. Therefore a diplexer with a terminated high pass filter at the input of the low pass filter may also be interposed between the driver output and amplifier input inside the RF generator.
Cascading filters such as those referenced in U.S. Pat. No. 5,187,457, are dissipative, yet are only suitable for fixed frequency plasma systems because of excessive power loss over the required bandwidth, removal of dissipated power and the associated size. Moreover, these filters lack sufficient rejection of the harmonics and so, in addition, require lossless filters for the desired plasma system harmonic rejection levels.
The semiconductor plasma processing equipment industry demands lower cost and much smaller size plasma generators, as fabrication space is now at a premium. High voltage MOSFETs with innovative circuit topologies, using surface mounted technology and improved cooling methods, have been considered as presenting a possible solution to meet this challenge. However, high voltage MOSFETs are sensitive to harmonic ground currents caused by switchmode driver output and energy reflected back from the plasma chamber. The occurrence of ground harmonic currents may cause the RF power amplifier to: a) switch inconsistently for the desired performance with respect to power gain and efficiency; b) become unstable and deliver incorrect power with respect to set point; c) cause plasma flux drop-out; and, d) increase gate-source differential voltages causing damage to the MOSFET dies.
These disadvantages of the prior art are overcome by the present invention.
The present invention is directed to a dual directional diplexer harmonics dissipation filter for a RF generator system which includes an input terminal coupled to a power source providing a radio frequency signal in a predetermined frequency range, an output terminal for providing the radio frequency signal to a load at the predetermined frequency range, a low pass filter having an input and output, the low pass filter connected between the input terminal and the output terminal, a plurality of high pass filters coupled to the low pass filter wherein the plurality of high pass filters dissipate signals in excess of the predetermined frequency range and the plurality of high pass filters have a predetermined circuitry effect selected from the group consisting of capacitance and inductance, and the resultant effect is offset and absorbed by the low pass filter at the input and the output.
Also disclosed is a dual directional harmonics dissipation filter for a frequency agile RF generator system powering a load, comprising an RF power amplifier, an output terminal coupled to the RF power amplifier providing a radio frequency signal in a predetermined frequency range, a low pass filter having an input and an output, an input impedance and an output impedance, the low pass filter coupled to the output terminal, a high pass filter coupled to the low pass filter output wherein the high pass filter receives and dissipates harmonic signals reflected from the load in excess of the predetermined frequency band and wherein the high pass filter""s effects on the output impedance of the low pass filter is offset, and filter means for managing harmonic signals produced by the RF power amplifier, the filter means coupled to the low pass filter input wherein the filter means"" effects on the input impedance of the low pass filter is offset.